As a result of prior art work in the treatment of bone fractures, it has been learned that a stable osteosynthesis at the fracture is obtained if the bone portions of the fracture can be secured and supported in a completely stable condition in compression or in distraction, with complete stability meaning that the bone portions involved in the fracture are held against any relative movement either longitudinally or laterally of the bone. Such prior art work has led to adoption of so-called intramedullary nails for fixation of tubular bones, and to the use of various types of stabilizing or supporting devices which operate within the medullary canal in the area of the fracture. In some such devices, the leading end of the intramedullary nail is split and the nail is equipped with a threaded rod, a threaded follower nut within the leading end of the nail operative to expand the split end, and an actuating nut and washer arrangement at the entrance of the medullary canal, as in U.S. Pat. No. 4,091,806, issued May 30, 1978 to Jacob Aginsky. In other devices, fingers carried by the nail are positively driven outwardly to engage in the wall of the medullary canal, as in U.S. Pat. No. 3,986,504, issued Oct. 19, 1976, to Rafall Avila. Though such prior art devices have achieved considerable success, there remain a number of problem areas which require further improvement.